Electro-mechanical resonant system



Feb. 3,

Filed Oct. 25, 1954 J. KAPLAN ET AL ELECTRO-MECHANICAL RESONANT SYSTEMTlc l.

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ELECTRO-MECHANICAL RESONANT SYSTEM Filed Oct. 25, 1954 2 Sheets-Sheet 2Tim 9:.

ATTORNEYS United States Patent Q ELECTRO-MECHANHCAJL RESONANT SYSTEM.iack Kaplan, Brooklyn, and Frank T. Turner, Hampton Bays, N. if.

Application October 25, 1954, Serial No. 464,520

12 Claims. (c1. sec-e6 The present invention relates toelectro-mechanical resonant systems and comprises a novel system of thistype which is self-excited and self-sustaining. As the invention isparticularly adapted for use in ultrasonic machining operations it willbe described with reference thereto. The principle of the invention isnot, however, limited to such specific application.

In ultrasonic machining, high frequency longitudinal vibrations are setup in a tool and tool holder attached to an electromechanical transducerand maximum amplitude of vibration at the working end of the too-l isobtained when the frequency is such that the overall length of tool,tool holder and transducer is approximately equal to an integral numberof half wavelengths of the compressional waves set up therein. Thus, foroptimum performance, the frequency of the electrical oscillationsdelivered to the transducer should be correlated to the mechanical partof the system. This means that the frequency of the electrical part ofthe system should change with tool and with tool wear, with change intemperature and with change in loading.

In prior art ultrasonic systems the electrical part of the systemordinarily included a variable frequency alternator or oscillator andattempts were made to adjust the frequency of such oscillator oralternator when a change in frequency was required to correlate theelectrical and mechanical parts of the system. Such adjustment of thefrequency of the oscillation generator Was done either by the operatoror automatically under control of a feedback signal obtained by use of apickup coupled to the mechanically vibrating part of the system. In suchprior art systems, whether manually controlled or automaticallycontrolled, a certain amount of hunting necessarily resulted. Moreover,when a pickup was employed extra equipment was required at thetransducer and the system was subject to error when high powered unitswere involved due to direct coupling between the energizing coil andpickup. Moreover, when a pickup is employed the spacing of parts iscritical as minute displacements can introduce errors into the system.

In accordance with the present invention the entire system including theelectromechanical transducer is so constructed as to comprise aself-excited oscillator with the transducer comprising part of thefrequency determining element of the oscillatory system. We have foundthat a system can be made to oscillate at or near the esired optimumfrequency when a feedback signal varying with th impedance of thetransducer is provided. The feedback signal may vary with the voltageacross the transducer or with the current through the transducer andprovided there is sufficient gain and that proper phase shift isintroduced into the feedback line the system will oscillate at thedesired frequency, the frequency automatically shifting with change inimpedance of the transducer indicative of tool wear or the like. Byutilizing the transducer impedance for creation of a feedback signal noexternal pick-up need be employed.

Fatented Feb. 3, 1959 For a better understanding of the invention and ofthe underlying theory thereof reference may be had to the accompanyingdrawings of which:

Fig. 1 is a qualitative representation of the complex electricalimpedance of a magnetostrictive transducer for use in explaining theinvention;

Fig. 2 is a circuit diagram representing one embodiment of theinvention; and

Fig. 3 is a circuit diagram representing another and simpler embodimentof the invention.

If a single mode of mechanical resonance is considered, the compleximpedance of a magnetostrictive transducer with increasing frequency maybe qualitatively represented by the curve A of Fig. l in which theordinates represent reactance and the abscissae represent resistance.

In the graph, harmonic frequencies are not represented and the reactiveimpedance of the transducer is taken to be primarily inductive, which isordinarily the case when the transducer is a magnetostrictivetransducer. A vector drawn from the origin to any point of the curve Arepresents the magnitude and phase angle of the impedance at thefrequency corresponding to that point. We have determined that thefrequency of maximum tool amplitude lies somewhere within the regionindicated by the bracket 2 on the loop B described by the curve A.Hence, by inserting in the feedback path of the electrical system avoltage varying with the magnitude of the impedance and of a phasecorrelated to the phase angle of the impedance at a point within thedesired range, oscillation of the system will result and the frequencywill be such as to insure operation of the system at or near resonance.

The circuit diagram of Fig. 2 to which reference may now be hadrepresents one embodiment of the invention as applied to a systemincluding a magnetostrictive transducer. In the drawing the transduceris diagrammatically illustrated at T. D. C. bias current from a suitablepower supply 4 is supplied to the transducer through a coil 6. Theungrounded end of the energizing winding of the transducer is connectedthrough a capacitor 3 to one end of the secondary of a transformer Itthe other end of which is connected to ground through an impedance 12.The primary winding of transformer ill is connected to the output of anamplifier 14, the input of which is connected across a resistor 16forming part of a phase shifting network. The phase shifting networkincludes in addition to the resistor 16 a variable capacitor 18 which isconnected in series with the resistor 16 across the output of anamplifier 20. One input terminal of the amplifier 20 is grounded and theother is connected by a lead 22 to the ungrounded end of impedance 12.Thus a voltage varying with the current through the transducer T isimpressed through the feedback connection 22 upon the amplifier 2G.

By proper adjustment of the capacity of capacitor 18 and of the biascurrent delivered to the transducer T through the coil r the circuit ofFig. 2 may be made to break into oscillation at or near the frequency ofmaximum amplitude of the transducer and the frequency of suchoscillation will automatically shift with change in transducerimpedance. Adjustment of capacitor 18 affects the phase of the feedbacksignal impressed upon ampliher 1 whereas adjustment of the bias currentaffects the impedance of the transducer and therefore both the phase rand magnitude of the feedback signal.

The circuit of Fig. 2 has been found very versatile in that it may beemployed with both high and low powered units and with transducers andtool holders of various dimensions.

The simpler circuit of Fig. 3 was designed with particular reference toa relatively small transducer, tool holder and tool combination, such,for example, as might be employed in a dental unit. The system of Fig. 3re" earners quires substantially less equipment than that of Fig.2 andis reliable and accurate in operation.

In the circuit of Fig. 3 the ungrounded end of the winding of transducerT is connected to one end of the secondary of 'a transformer 24 and theother end of that secondary is connected through a capacitor 26 and theprimary winding of a transformer 28 to ground. The secondary oftransformer 28 is grounded at its mid point and connected at its ends tothe control grids of two triodes 3t and 32. The anodes of the triodes3t) and 32 are connected across the primary of transfer- :1 Thus theprimary Winding of transformer 28 carries the transducer current and thevoltage thereacross will vary with the current through the transducer.As in the circuit of Fig. 2, D. C. bias current for the transducer issupplied through the coil 6. In Fig. 3 the power ply for the D. C. biascurrent comprises a pair of rectifiers 34 connected to opposite ends ofthe center grounded secondary of a transformer 36, the primary of whichis connected to an auto transformer 38 energized from 115 volt 60 cyclepower lines dd under control if a switch 42. Rectified and filteredvoltage for the anodes of tubes and 32 is obtained from a rectifier 44energized from the power lines through a transformer 46 and filamentcurrent for tubes 30 and 32 is obtained from one secondary winding of atransformer 48, the primary of which is also energized from the powerlines 4%. A separate secondary winding on transformer 48 supplies thefilament current for the rectifier 44. The primary of transformer 46 isconnected across the power lines 49 through an additional switch 59which may be, and preferably is, a foot operated switch. Thus the supplyof oscillatory current to the transducer may be cut off at will by anoperator manipulating switch 50 and this is of particular value when thetransducer forms part of an ultrasonic dental tool. In ultrasonicmachining, as described in Patent No. 2,580,716 to Lewis Balamuth,cutting is done by an abrasive suspended in liquid and interposedbetween the vibrating tool'end and the workpiece. When the transducer isa magnetostrictive transducer it may be desired to circulate coolingwater thereabout to prevent overheating. A indicated in Fig. 3 a pumpfor circulating the cooling fluid and/ or the fluid carrying theabrasive can be energized from lines 52 and 54 connecting transformer 38to the main switch 42.

In operation, assuming no major variation in dimensions of themechanical part of the system, the only adjustment required to start andmaintain oscillation of the circuit of Fig. 3 is that of the biascurrent, and such may be made by change in position of the movable tapon auto-transformer 38, or, if preferred, a variable resistor could beinserted in the line from the rectifiers 34 to the coil 6. If thedifferences in the tool and tool holder assemblies to be used with thesystem of Fig. 3 are substantial, the windings of transformers 28 and 24could be provided with taps for selection of optimum numbers of turnsfor the primary and secondary windings thereof and means could beprovided for replacing capacitor 26 by one or more different capacitors.

From the foregoing description of two embodiments of the invention itwill be apparent that we have provided a simple and efficientself-excited electro-mechanf.

cal system in which oscillations are automatically sustained at or nearresonance irrespective of change in dimensions or of loading of themechanical part of the system. In each case the electro-mechanicaltransducer determines in part the frequency of the system and a voltagevarying with the electrical impedance thereof is fed back to provide theoutput-input loop for initiating and sustaining oscillations. Althoughin the specific. embodiments of the invention illustrated in thedrawings, the current through the transducer was utilized to produce thefeedback voltage,.the invention in its broadest aspects is not limitedto such specific arrangement. Also,

although in the specific embodiments illustrated the power output wasshown as fixed, obviously means could be provided for adjusting orvarying the power output of the circuit. Preferably D. C. bias currentis employed and therefore in the circuits of Figs. 2 and 3 directcurrent sources for the bias current were indicated. Nevertheless theinvention is not limited to the use of such type of bias current asalternating current bias current could be employed.

In the described specific embodiments of the inventicn, th ive impedanceof the transducer is primarily inductive and therefore the feedback pathincludes capacitative phase shift means. Obviously, when the reactiveimpedance of the electro-mechanical transducer is primarilycapacitative, inductive phase shift means could .vould beincluded in thefeedback path. Although the invention has'been described with specificreference to ultrasonic machining, the principle thereof is notdependent upon any particular field of utility of the generatedmechanical vibrations.

The following is claimed:

1. An electromechanical resonant system comprising a an output ansformerhaving primary and secondary windings, the primary winding of saidoutput transformer being connected across the output terminals of saidamplifier, means coupiing the secondary of said output transformer tothe energizing winding of said transducer, and feedback and phaseshifting means for impressing across the input terminals of saidamplifier a voltage varying with the current through the winding of saidtransducer to cause oscillation of the system substantially -atresonance.

2. A self-starting electro-mechanical resonant system comprising asource or" electrical power, a magnetostrictive transducer having anenergizing winding, adjustable means for delivering bias current fromsaid source to said winding, an amplifier energized from said source andhaving input and output terminals, an input transformer having primaryand secondary winding, said secondary winding being connected across theinput terminals of said amplifier, an output transformer having aprimary and a secondary winding, the primary winding of said outputtransformer being connected across the output terminals of saidamplifier, and a circuit including phase shifting means connecting oneend of the primary winding of said input transformer throughthesecondary winding of said output transformer to one end of saidtransducer winding, the other ends of said transducer winding and inputtransformer primary winding being connected together.

3. The system according to claim 2 wherein the reactive impedance ofsaid transducer is primarily inductive and the phase shifting means insaid connecting circuit comprises a capacitor.

4. The system according to claim 3 wherein said capacitor is interposedbetween the primary winding of said input transformer and the secondarywinding of the output transformer.

5. A self-starting electro-mechanical resonant system comprising asource of electrical power, a magnetostrictive transducer having anenergizing winding, means for delivering bias current from said sourceto said winding, an amplifier energized from said source and havinginput and output terminals, an output transformer having a primary and asecondary winding, th primary winding of said ou t transformer beingconnected across the output terminals of said amplifier, the secondarywinding of said output transformer being connected at one end through acapacitor to one end of said transducer wind and at its other end toground through an impedance, and circuit means'for impressing across theinput terminals of said amplifier a voltage varying with the voltageacross said impedance, said circuit means including amplifying and phaseshifting means.

6. The system according to claim 5 wherein the reactive impedance ofsaid transducer is primarily inductive and the phase shifting means ofsaid circuit means comprise a variable capacitor.

7. The system according to claim 5 wherein said circuit means comprisean amplifier having input terminals connected across said impedance andoutput terminals connected across a series circuit comprising a variablecapacitor and a resistor, said last mentioned amplifier comprising theamplifying means of said circuit means and said variable capacitorcomprising the phase shifting means, the input terminals of said firstmentioned amplifier being connected'across said resistor.

8. A self-excited oscillatory system for acoustical power generationincluding in combination, power amplifying means, an electro-mechanicaltransducer having a single energizing winding driven by said poweramplifying means and serving as a part of the frequency determiningelement of the system, means connected to the input end of said windingoperative to feed back to the input of the power amplifying means avoltage varying with the current through the transducer to therebymaintain oscillation of the system substantially at resonance, and meansfor shifting the phase of the feedback voltage.

9. An electro-mechanical resonant system including, a magnetostrictivetransducer having a single energizing winding and an electric poweramplifying circuit connected to said winding and operative to supply abias current to said winding which consists of a direct currentcomponent having a superimposed alternating current component, saidpower amplifying circuit having means connected to the input end of saidwinding operative to feed back through said winding a voltage whichvaries with the alternating current component or" said bias current andthereby maintain the oscillations of the system substantially atresonance.

10. An electro-mechanical resonant system including, a magnetostrictivetransducer having a single energizing winding and an electric powercircuit connected to said winding and operative to supply a bias currentto said winding, said power circuit including means connected to theinput end of said winding operative to feed back through said winding avoltage varying with said current and thereby maintain oscillation ofthe system substantially at resonance, and adjustable means for varyingthe impedance of said transducer to thereby vary the magnitude and shiftthe phase of the feedback voltage.

11. A self-excited oscillatory system for acoustical power generationincluding in combination, power amplifying means, an electro-mechanicaltransducer having a single energizing winding driven by said poweramplifying means and serving as a part of the frequency determiningelement of the system, and means connected to the input end of saidwinding operative to feed back to the input of said amplifying means avoltage which varies with the impedance of the transducer to therebymaintain oscillation of the system substantially at resonance.

12. A self-excited oscillatory system for acoustical power generationincluding in combination, power amplifying means, an electro-mechanicaltransducer having a single energizing winding driven by said poweramplifying means and serving as a part of the frequency determiningelement of the system, and means connected to the input end of saidwinding operative to feed back to the input of the amplifying means avoltage varying with the current through the transducer to therebymaintain oscillation of the system substantially at resonance.

References (Iited in the file of this patent UNITED STATES PATENTS1,311,128 Harrison June 23, 1931 1,937,333 Dome Nov. 28, 1933 1,982,341Hitchcock Nov. 27, 1934 1,992,938 Chambers et a1. Mar. 5, 1935 2,396,224Artzt Mar. 12, 1946 2,676,236 Birkbeck et a1 Apr. 20, 1954 2,683,856Kornei July 13, 1954

